Plasmodium sp. is the etiologic agent causing malaria, also known as “paludism”. There are different species within the Plasmodium genus, some of which are innocuous. Other species, on the contrary, are highly infectious and are the cause of most of the human malaria cases worldwide. Among the latter, the most important species are P. falciparum, and P. vivax. 
All the human Plasmodium species (P. falciparum, P. vivax, P. malariae, P. ovale, P. knowlesi) infect, to a greater or lesser extent, erythrocytes and/or the precursors thereof, reticulocytes, which require a process of maturation and differentiation to reach their final functional state as erythrocytes. Among the different Plasmodium species, there are some which have a higher reticulocyte infection capacity than others, such as Plasmodium vivax for example, which predominantly infects cells of this type.
During the process of maturation and differentiation of reticulocytes into erythrocytes, some proteins, which are not necessary for the latter, are sequestered in internal vesicles located in multi-vesicular bodies (MVBs) and are subsequently released into the extracellular medium as small nano-vesicles known as exosomes.
Recently, research on exosomes has been stimulated after the discovery that other cells, such as antigen-presenting cells, are capable of secreting nano-vesicles of this type, suggesting a role beyond the one originally described in the maturation and differentiation of reticulocytes. In fact, several studies with different types of cells have revealed that exosomes play a role in the regulation of the immune system since they transfer information between cells during immune response, and therefore, represent a new way of intercellular communication (1). In this line, the protective capacity of exosomes in experimental infections with Toxoplasma gondii, a member of the Apicomplexa phylum to which Plasmodium also belongs has been demonstrated (2).
Furthermore, several strategies based on the use of exosomes as prophylactic or immunostimulating agents for humans have been described (3).
Among others, WO9705900 discloses exosomes obtained from antigen-presenting cells such as B cells, macrophages or dendritic cells. The exosomes described in this document have the particularity that, since they are obtained from antigen-presenting cells, the antigens are presented in the MHC-I and MHC-II context.
Document EP1523990 in turn describes exosomes obtained from cancer cells (identified as texosomes) or from dendritic cells loaded or unloaded with antigens (this document refers to them as dexosomes).
WO2004014954 uses a different strategy since, in order to obtain exosomes showing a desired antigen in its surface, cells from the line CT26 (murine colon cancer) and cells from the line TA3HA (mouse mammary carcinoma) are transfected with recombinant viruses comprising in their genome the sequence which encodes the desired antigen (muc-1), which is thus expressed in the surface of the exosomes isolated from cells of this type.
WO0028001 describes exosomes obtained from mastocytes essentially lacking endogenous MHC molecules. The exosomes described in this document do express, however, recombinant MHCs in their surface.
Finally, WO2008092153 describes exosomes obtained from cancer cells which lack one or more immunosuppressive polypeptides normally present in exosomes.
The authors of the present invention have now developed a new strategy in the prevention and prophylaxis of malaria based on the use of exosomes isolated from reticulocytes of murine models or subjects infected with Plasmodium. These exosomes are obtained from peripheral blood infected with Plasmodium sp. or from in vitro cultures of reticulocytes previously obtained from said peripheral blood.